Abstract:

An LED semiconductor element comprising at least one first
radiation-generating active layer and at least one second
radiation-generating active layer which is stacked above the first active
layer in a vertical direction and is connected in series with the first
active layer, wherein the first active layer and the second active layer
are electrically conductively connected by means of a contact zone.

Claims:

1.-39. (canceled)

40. An LED semiconductor element comprising at least one first
radiation-generating active layer and at least one second
radiation-generating active layer which is stacked above the first active
layer in a vertical direction and is connected in series with the first
active layer, whereinthe first active layer and the second active layer
are electrically conductively connected by means of a contact zone.

41. The LED semiconductor element as claimed in claim 40, wherein the
contact zone is arranged at a side flank of the LED semiconductor
element.

42. The LED semiconductor element as claimed in claim 40, wherein a
semiconductor layer of a first conductivity type is disposed downstream
of the first active layer in a vertical direction.

43. The LED semiconductor element as claimed in claim 42, whereina
semiconductor layer of a second conductivity type is arranged between the
semiconductor layer of the first conductivity type and the second active
layer.

44. The LED semiconductor element as claimed in claim 43, whereinthe
semiconductor layer of the first conductivity type and the semiconductor
layer of the second conductivity type form a tunnel junction.

45. The LED semiconductor element as claimed in claim 43, whereinthe
semiconductor layer of the first conductivity type comprises a first free
region not covered by semiconductor material.

46. The LED semiconductor element as claimed in claim 45, whereinthe
semiconductor layer of the second conductivity type comprises a second
free region not covered by semiconductor material.

47. The LED semiconductor element as claimed in claim 46, whereinthe
contact zone extends from the first free region to the second free
region.

48. The LED semiconductor element as claimed in claim 40, whereinthe
contact zone contains a TCO.

49. The LED semiconductor element as claimed in claim 40, whereinthe
contact zone is integrated into the LED semiconductor element between the
first active layer and the second active layer.

50. The LED semiconductor element as claimed in claim 49 whereinthe
contact zone has at least one first region and at least one second
region.

51. The LED semiconductor element as claimed in claim 50, wherein the
second region is electrically conductive.

52. The LED semiconductor element as claimed in claim 49, whereinthe
contact zone contains a material that is transmissive to the radiation
generated by the first and/or second active layer.

53. The LED semiconductor element as claimed in claim 52, whereinthe
contact zone contains a TCO.

Description:

[0001]The invention relates to an LED semiconductor element and to the use
of an LED semiconductor element.

[0002]This patent application claims the priority of German patent
application 102006046039.1 and the priority of German patent application
102006051745.8, the disclosure content of which is hereby incorporated by
reference.

[0003]A high luminance is desirable for optical applications such as
projection applications or display backlightings. In conventional LED
semiconductor elements, the amount of radiation generated depends on the
current intensity with which the LED semiconductor element is operated.
However, the current density in the active layer should not exceed a
maximum current density dependent on the semiconductor material used,
since otherwise there is the risk of excessive ageing effects
disadvantageously shortening the lifetime of the LED semiconductor
element.

[0004]It is an object of the present invention to specify an LED
semiconductor element having an increased luminance.

[0005]This object is achieved by means of an LED semiconductor element in
accordance with patent claim 1.

[0006]Moreover, it is an object of the present invention to specify uses
of an LED semiconductor element with increased luminance.

[0007]This object is achieved by means of uses in accordance with patent
claims 36 to 39.

[0008]The dependent claims relate to advantageous developments and
configurations of the invention.

[0009]An LED semiconductor element according to the invention comprises at
least one first radiation-generating active layer and at least one second
radiation-generating active layer which is stacked above the first active
layer in a vertical direction and is connected in series with the first
active layer, wherein the first active layer and the second active layer
are electrically conductively connected by means of a contact zone.

[0010]In the present case, the contact zone should be understood to be a
region of comparatively good electrical conductivity, wherein the contact
zone is preferably embodied in a manner free of tunnel contacts and
therefore does not constitute a tunnel junction. Moreover, in the LED
semiconductor element according to the invention, no tunnel junction is
required for a charge carrier transfer between the first and the second
active layer. This has the advantage that the LED semiconductor element
can also be produced from materials with which a tunnel junction is
relatively difficult to realize epitaxially. Although the active layers
could be connected in parallel, such that a tunnel junction would be
superfluous, a parallel connection would have the disadvantage that with
different series resistances, the same current could not be injected into
the two active layers, or could be injected only with considerable
additional outlay. It is advantageously possible according to the
invention to provide for a sufficient charge carrier transfer between the
first and the second active layer by means of the contact zone and
furthermore to inject the same current into the two active layers by
means of the series connection.

[0011]In the present series connection, the pn junctions of the active
layers are preferably arranged in the same sense, such that they form a
pn-pn or np-np structure. It goes without saying that in the case of more
than two active layers, a pn . . . pn or np . . . np structure is
preferred.

[0012]Apart from a simple pn junction, the active layers can have a double
heterostructure, a single quantum well or a multiple quantum well (MQW)
structure. Examples of MQW structures are described in the documents WO
01/39282, WO 98/31055, U.S. Pat. No. 5,831,277, EP 1 017 113 and U.S.
Pat. No. 5,684,309, the disclosure content of all of which concerning the
MQW structures is hereby incorporated by reference.

[0013]In particular, two arrangements of the contact zone are preferred in
the context of the invention. In accordance with a first arrangement, the
contact zone is arranged at a side flank of the semiconductor element. In
accordance with a second arrangement, the contact zone is integrated into
the LED semiconductor element between the first active layer and the
second active layer.

[0014]Since the first active layer and the second active layer are
connected in series, in the case of the active layers being arranged in
the same sense, the contact zone expediently connects a semiconductor
layer of a first conductivity type to a semiconductor layer of a second
conductivity type. Preferably, the semiconductor layer of the first
conductivity type is disposed downstream of the first active layer in a
vertical direction, while the semiconductor layer of the second
conductivity type is arranged in a vertical direction between the
semiconductor layer of the first conductivity type and the second active
layer. By way of example, the semiconductor layer of the first
conductivity type can be a p-doped semiconductor layer and the
semiconductor layer of the second conductivity type can be an n-doped
semiconductor layer. As an alternative, the semiconductor layer of the
first conductivity type can be an n-doped semiconductor layer and the
semiconductor layer of the second conductivity type can be a p-doped
semiconductor layer. This depends on the arrangement of the pn junctions
of the active layers.

[0015]In order to improve the charge carrier transfer in a semiconductor
element whose contact zone is arranged at the side flank, the
semiconductor layer of the first conductivity type and the semiconductor
layer of the second conductivity type can form a tunnel junction that
supports the charge carrier transfer in addition to the contact zone. In
particular, the semiconductor layer of the first conductivity type and
the semiconductor layer of the second conductivity type can be highly
doped for this purpose.

[0016]In accordance with one preferred embodiment, the semiconductor layer
of the first conductivity type comprises a first free region not covered
by semiconductor material. With further preference, the semiconductor
layer of the second conductivity type comprises a second free region not
covered by semiconductor material. In particular, the semiconductor layer
of the second conductivity type can project with respect to the rest of
the semiconductor element, while the semiconductor layer of the first
conductivity type projects with respect to the layer of the second
conductivity type. Consequently, the form of the semiconductor element
between the first active layer and the second active layer in cross
section can correspond to a stepped form at least at a side flank. It
should be pointed out that the LED semiconductor element has a layer
sequence of layers, of which at least a portion contains a semiconductor
material. In the present case, the free region not covered by
semiconductor material should be understood to be a region not covered by
a semiconductor material used for the layers of the layer sequence.

[0017]In accordance with a particularly preferred variant, the contact
zone extends from the first free region to the second free region. In
particular, the contact zone can be a contact layer. By way of example,
the contact layer can at least partly cover the first free region and the
second free region.

[0018]Materials used for the contact zone and dimensions of the contact
zone are preferably chosen depending on the lateral conductivity of the
layers which the contact zone electrically conductively connects. By way
of example, a p-doped GaN layer has a relatively low lateral
conductivity, for which reason the contact zone in this case should be
embodied in comparatively large-area fashion and should contain a
material having high electrical conductivity.

[0019]In accordance with one preferred embodiment of the LED semiconductor
element, the contact zone contains a metallic material. Such a contact
zone is distinguished by a comparatively good electrical conductivity.
This advantageously facilitates the charge carrier transfer between the
first active layer and the second active layer.

[0020]In an alternative or more extensive configuration of the LED
semiconductor element, the contact zone can be formed from a TCO
(transparent conductive oxide) such as indium oxide, tin oxide, indium
tin oxide (ITO) or zinc oxide.

[0021]A contact zone containing a TCO is advantageously
radiation-transmissive, such that the radiation generated in a region
below the contact zone can be coupled out from the semiconductor element
through the contact zone.

[0022]In accordance with one advantageous configuration, the first and the
second active layer are monolithically integrated in the semiconductor
element. In this case, the first and the second active layer can be
produced in a common production step.

[0023]Furthermore, the semiconductor element in the case of the invention
can be a thin-film semiconductor element. If the semiconductor element is
composed of prefabricated layer stacks, then the individual layer stacks
can be thin-film semiconductor bodies. A thin-film semiconductor element
is distinguished in particular by at least one of the following
characteristic features: [0024]a reflective layer is applied or formed
at a first main area--facing toward a carrier element--of a
radiation-generating epitaxial layer sequence, said reflective layer
reflecting at least part of the electromagnetic radiation generated in
the epitaxial layer sequence back into the latter; [0025]the epitaxial
layer sequence has a thickness in the range of 20 μm or less, in
particular in the range of between 2 μm and 10 μm; and [0026]the
epitaxial layer sequence contains at least one semiconductor layer having
at least one area having an intermixing structure which ideally leads to
an approximately ergodic distribution of the light in the epitaxial layer
sequence, that is to say that it has an as far as possible ergodically
stochastic scattering behavior.The basic principle of a thin-film light
emitting diode chip is described for example in I. Schnitzer et al.,
Appl. Phys. Lett. 63 (16), Oct. 18, 1993, 2174-2176, the disclosure
content of which in this respect is hereby incorporated by reference.

[0027]A thin-film semiconductor element is, to a good approximation, a
Lambertian surface emitter and is therefore particularly well suited to
projection applications.

[0028]As already mentioned, in the case of the second arrangement the
contact zone is integrated into the LED semiconductor element between the
first active layer and the second active layer.

[0029]In accordance with one preferred configuration, the semiconductor
element comprises a first layer stack including the first active layer,
and a second layer stack including the second active layer. Particularly
preferably, the contact zone is embedded between the first layer stack
and the second layer stack in the case of this configuration.

[0030]In particular, the first layer stack comprises a semiconductor layer
of a first conductivity type in addition to the first active layer and
the second layer stack comprises a semiconductor layer of a second
conductivity type in addition to the second active layer. Preferably, the
first and the second layer stack are produced from two individual wafers.
In order to produce the semiconductor element according to the invention,
the wafers can be bonded onto one another in such a way that the
semiconductor layer of the first conductivity type and the semiconductor
layer of the second conductivity type face one another.

[0031]In accordance with a configuration to which further preference is
given, the contact zone is arranged between the semiconductor layer of
the first conductivity type and the semiconductor layer of the second
conductivity type. Consequently, the contact zone is arranged in the main
beam path of the semiconductor element, while in the first arrangement
the contact zone is arranged in particular outside the main beam path.

[0032]In accordance with one preferred embodiment, the contact zone is a
contact layer.

[0033]In accordance with an embodiment to which further preference is
given, the contact zone has at least one first region and at least one
second region. Particularly preferably, the second region is electrically
conductive. The first region can be electrically conductive or
insulating. By way of example, the contact zone can comprise a second
region in the form of a contact pad or elongated contact web, wherein a
material surrounding the second region forms the first region. The second
region is advantageously arranged in such a way that it produces an
electrical connection between the first and the second layer stack. In
particular, the second region can contain a metallic material. The second
region is preferably applied on a surface of the first or second layer
stack which faces the opposite layer stack. As an alternative, each layer
stack can have at least one second region arranged in such a way that in
each case two second regions come to lie on one another when the two
layer stacks are stacked one on another.

[0034]The contact zone expediently contains a material that is
transmissive to the radiation generated by the first and/or second active
layer. Consequently, there is no need to fear significant radiation
losses through the contact zone arranged in the main beam path.

[0035]The contact zone can contain a TCO. Furthermore, the contact zone
can contain an adhesion agent.

[0036]Furthermore, a first connecting layer can be applied on the
semiconductor layer of the first conductivity type and a second
connecting layer can be applied on the semiconductor layer of the second
conductivity type. The connecting layers can be provided in particular
for further improving the charge carrier transfer between the layer
stacks. Preferably, the connecting layers contain a
radiation-transmissive and electrically conductive material such as TCO.
Particularly preferably, the contact zone is arranged between the first
connecting layer and the second connecting layer.

[0037]Furthermore, a mechanical connection is advantageously produced by
means of the contact zone between the first and second layer stacks.

[0038]Preferably, the first and the second active layer generate radiation
having the same wavelength. The amount of radiation is thus
advantageously increased by comparison with conventional LED
semiconductor elements.

[0039]With further preference, the main emission from the LED
semiconductor element is effected in a vertical direction. In particular,
the main emission is effected within a comparatively constricted solid
angle, such that the luminance is advantageously increased. The luminance
is the optical power per emission area of the semiconductor element and
solid angle element.

[0040]Particularly preferably, the radiation generated by the first active
layer radiates through the second active layer. This is advantageous
particularly in combination with a reflection layer that can be provided
for the reflection of the radiation generated by the active layers in a
vertical direction. For in contrast to active layers which generate
radiation of different wavelengths, in this case the absorption of
reflected radiation by the respective other active layer has no
disadvantageous effect on the total radiation emitted.

[0041]In accordance with one variant, the semiconductor element,
preferably one of the two active layers or both active layers, contains
AlnGamIn.sub.1-n-mP, where 0≦n≦1,
0≦m≦1 and n+m≦1.

[0042]In accordance with a further variant, the semiconductor element,
preferably one of the two active layers or both active layers, contains
AlnGamIn.sub.1-n-mAs, where 0≦n≦1,
0≦m≦1 and n+m≦1.

[0043]In accordance with a further variant, the semiconductor element,
preferably one of the two active layers or both active layers, contains
AlnGamIn.sub.1-n-mN, where 0≦n≦1,
0≦m≦1 and n+m≦1.

[0044]The LED semiconductor element according to the invention can
advantageously be used for a radiation-emitting component since high
luminances in conjunction with a comparatively small component size can
be obtained by means of the LED semiconductor element.

[0045]Furthermore, the LED semiconductor element according to the
invention or the radiation-emitting component comprising the LED
semiconductor element according to the invention can be used in
particular for general lighting, for backlighting, for example of
displays, or for projection applications.

[0046]Further features, advantages and developments of the invention will
become apparent from the exemplary embodiments explained below in
conjunction with FIGS. 1 to 4.

[0047]In the figures:

[0048]FIG. 1 shows a schematic cross-sectional view of a first exemplary
embodiment of an LED semiconductor element according to the invention,

[0049]FIG. 2 shows a schematic cross-sectional view of a second exemplary
embodiment of an LED semiconductor element according to the invention,

[0050]FIG. 3 shows a schematic cross-sectional view of a third exemplary
embodiment of an LED semiconductor element according to the invention,

[0051]FIG. 4 shows a schematic cross-sectional view of a fourth exemplary
embodiment of an LED semiconductor element according to the invention.

[0052]The LED semiconductor element 1 in accordance with a first exemplary
embodiment as illustrated in FIG. 1 comprises a first
radiation-generating active layer 2 and a second radiation-generating
active layer 3, wherein the active layers are arranged one above another
in a vertical direction, that is to say parallel to an emission direction
and perpendicular to a main extension direction of the active layers. A
first semiconductor layer 5 of a first conductivity type, for example a
p-conducting semiconductor layer, and a second semiconductor layer 6 of a
second conductivity type, for example an n-conducting semiconductor
layer, are arranged between the active layers 2, 3.

[0053]The arrangement of the two active layers 2, 3 in the LED
semiconductor element 1 advantageously increases the amount of radiation
generated overall. Since the dimensions of the LED semiconductor element
1 change only insignificantly by comparison with an LED semiconductor
element having only a single active layer and, in particular, the cross
section of the LED semiconductor element is independent of the number of
active layers, more extensively the luminance is also advantageously
increased.

[0054]The semiconductor element 1 comprises a contact zone 4, which
electrically conductively connects the semiconductor layer 5 to the
semiconductor layer 6. Preferably, the semiconductor element 1 is
processed in such a way that on at least one side flank part of the
semiconductor layer 5 and part of the semiconductor layer 6 are
uncovered, whereby a first free region 9 not covered by semiconductor
material and a second free region 10 not covered by semiconductor
material are formed. The contact zone 4 extends from the first free
region 9 to the second free region 10 and at least partly covers them.
The contact zone 4 can contain a metal, a metal compound or a
radiation-transmissive oxide (TCO) such as ITO.

[0055]Furthermore, in order to improve the electrical connection, the two
semiconductor layers 5, 6 can be embodied in highly doped fashion, such
that an efficient tunnel junction with a low electrical contact
resistance arises during operation.

[0056]The LED semiconductor element 1 comprises a rear side contact 7
disposed upstream of the active layers 2 and 3 in the vertical direction.
Furthermore, the LED semiconductor element 1 comprises a front side
contact 8 disposed downstream of the active layers 2 and 3 in a vertical
direction. Consequently, a vertically conductive component is formed
which is distinguished by a comparatively homogeneous current
distribution within the LED semiconductor element 1.

[0057]More extensively, the semiconductor element 1 can be arranged on a
carrier element (not illustrated) on the side of the rear side contact 7.
In this case, the carrier element preferably contains an electrically
conductive material. By way of example, the semiconductor element 1 can
be a thin-film semiconductor element. In this case, the LED semiconductor
element 1 is grown in particular on a growth substrate different than the
carrier element and is subsequently mounted onto the carrier element,
which can be done for example by means of soldering, bonding or adhesive
bonding, the growth substrate preferably being stripped away from the LED
semiconductor element. The rear side contact 7 can simultaneously serve
as a mirror, such that radiation components impinging on the rear side
contact 7 are reflected in a vertical direction, that is to say in this
case in the direction of a radiation coupling-out side of the LED
semiconductor element 1.

[0058]In the exemplary embodiment illustrated in FIG. 1, the active layers
2 and 3 are preferably monolithically integrated in the semiconductor
element 1. By contrast, in the exemplary embodiment illustrated in FIG.
2, an individual first layer stack I comprising the active layer 2 and an
individual second layer stack II comprising the active layer 3 are
connected to one another in order to obtain the LED semiconductor element
1. The production step of connecting the two layer stacks I and II is
symbolized by the arrows.

[0059]A contact layer that forms the contact zone 4 after the connection
of the two layer stacks I and II is arranged on the layer stack I. As an
alternative, the contact layer can be arranged on the layer stack II. The
contact zone 4 is subsequently integrated into the LED semiconductor
element 1 between the first active layer 2 and the second active layer 3.
The contact layer contains an electrically conductive material.
Furthermore, the contact layer is transmissive to the radiation of the
active layer 2 and/or of the active layer 3. The contact layer preferably
contains an adhesion agent, such that the two layer stacks I and II are
mechanically connected by means of the contact layer.

[0060]A rear side contact 7 can be applied to the layer stack I, while a
front side contact 8 can be formed on the layer stack II. The contacts
can be applied before or after the connection of the two layer stacks I
and II.

[0061]In the exemplary embodiment of an LED semiconductor element 1 which
is illustrated in FIG. 3, two layer stacks I and II are likewise arranged
one on top of the other, wherein the contact zone 4 is integrated into
the LED semiconductor element 1 between the layer stacks I and II. The
contact zone 4 comprises a first region 4a and a plurality of second
regions 4b. The second regions 4b are embedded into the first region 4a.
Preferably, the second regions 4b are electrically conductive. The region
4a can be electrically conductive or insulating. As illustrated, the
second regions 4b can be embodied in the form of contact pads, wherein
one second region 4b is arranged on the layer stack I and another second
region 4b is arranged on the layer stack II The two layer stacks I and II
are connected to one another in such a way that the two regions 4b lie
one on another. The layer stacks I and IT are electrically conductively
connected to one another by means of the second regions 4b. Furthermore,
the active layers 2 and 3 and the contact zone 4 are arranged with
respect to one another in such a way that the active layers 2 and 3 are
connected in series.

[0062]The two layer stacks I and II can be bonded onto one another by
means of the second regions 4b. In addition, the first region 4a can
contain an adhesion agent that mechanically connects the two layer stacks
I and II. Preferably, the first region 4a is transmissive to the
radiation generated by the active layer 2 and/or the active layer 3.

[0063]The LED semiconductor element 1 illustrated in FIG. 4 comprises a
first layer stack I and a second layer stack II disposed downstream of
the first layer stack I in a vertical direction, wherein the contact zone
4 is arranged between the layer stack I and the layer stack II. The
contact zone 4 comprises a first region 4a and a second region 4b. The
first region 4a and the second region 4b are arranged between a first
connecting layer 4c and a second connecting layer 4d. The first
connecting layer 4c and the second connecting layer 4d preferably serve
to improve the charge carrier transfer between the layer stacks I and II.
By way of example, the connecting layers 4c and 4d can contain a
radiation-transmissive electrically conductive oxide (TCO) such as ITO.
Particularly preferably, the connecting layers 4c and 4d are applied to
the respective layer stack before the connection of the two layer stacks
I and II. One of the two layer stacks I and II is furthermore provided
with the second region 4b, embodied in particular as a contact pad or
contact web. The second region 4b contains a material, in particular a
metal, having a low electrical resistance, such that a comparatively good
current flow across the contact zone 4 can take place. The first region
4a preferably contains an adhesion agent, such that the layer stacks I
and II are mechanically connected in particular by means of the first
region 4a.

[0064]The invention is not restricted by the description on the basis of
the exemplary embodiments. Rather, the invention encompasses any new
feature and also any combination of features, which in particular
comprises any combination of features in the patent claims, even if this
feature or this combination itself is not explicitly specified in the
patent claims or exemplary embodiments.